Elimination of so-called "hotmelt" coatings from paper mill "broke" and trimmings or from used consumer items, such as milk cartons, frozen food boxes and the like is essential to recycling of the high quality cellulose fibers generally contained in such paperboard.
A number of processes employing various aqueous and nonaqueous solvents, separately or in combination, have been developed for the purpose of removing hotmelt coatings. None of these processes employ vapor pools of non-aqueous solvents. The process presently used commercially is the so-called "Polysolv" process, described in U.S. Pat. No. 3,058,871. This patent constitutes the closest known prior art.
In the latter process, shredded "furnish" is washed with progressively cleaner batches of a super-heated liquid solvent, such as trichloroethylene, in a rotary extractor or autoclave. The denuded furnish is then stripped of residual solvent, in the extractor, with steam. The "dirtiest" solvent is distilled and the bottoms are mixed with fuel oil and further distilled to remove the last of the solvent. The residual polymer/fuel oil mixture is incinerated or utilized as boiler fuel.
U.S. patent application Ser. No. 371,836, published in abstract in the Jan. 28, 1975 Official Gazette as part of a Trial Voluntary Protest Program, is directed to the method of cleaning an article contaminated with organic acid materials so as to form a biodegradeable waste comprising the steps of:
inserting the article to be cleaned of organic acid residues into the vapor section of a vapor degreaser, said vapor degreaser having a two-phase liquid solvent located in the sump thereof, said two-phase liquid solvent comprising a first liquid containing an organic solvent and a second liquid containing water and an inorganic base of low volatility, said inorganic base being sufficiently strong to neutralize said organic acid materials, and said first and second liquids being immiscible in one another; and PA1 heating said mixture to boil said two-phase liquid solvent wherein said solvent is in a state of turbulence and a vapor of water and organic solvent is created in said vapor section and refluxing said mixture, without vaporizing said inorganic base. PA1 (a) maintaining a pool of hot, saturated vapors of a halogenated, acyclic hydrocarbon containing from one to three carbons and from two to eight bromine, chlorine or fluorine atoms, at least two of which are bromine or chlorine, PA1 (b) placing pieces of the coated paperstock, which are at a temperature below the condensation temperature of said vapors, in said vapor pool, thereby causing formation of a continuous, off-running film of condensate on the resin, PA1 (c) agitating together said pieces of paperstock in the vapor pool until at least a predominant proportion of said resin has been removed by the combined actions thereon of said agitation and of the vapors and condensate. PA1 (1) said resin is converted to a semi-solid or gel phase as a result of swelling and softening by the hot vapors, PA1 (2) the combined actions of the condensate and gravity cause said phase to separate from said paperstock and fall, and PA1 (3) said phase is intercepted and recovered essentially free of any of said halogenated hydrocarbon present as a separate liquid phase. PA1 (1) Shredding of the coated paperstock into strips of a width and length appropriate to the apparatus selected. PA1 (2) Agitating together the strips while exposing them to the hot, saturated, halogenated hydrocarbon vapors until a desired proportion of the coating has been removed by the combined actions of the vapors, the condensate and gravity. Usually, the shredded stock will be kept in the vapors at least until condensation on the shreds no longer occurs, resulting in essentially complete removal of the solvent-wettable coating resin. For example, from 15 seconds to 2 minutes usually suffices when ordinary hot-melt coated paperboard is immersed in hot vapors of perchloroethylene. PA1 (3) Interception or separation of any resinous materials flushed from the paperstock by the condensate as a discrete phase, such as a solution gel or a melt. PA1 (4) A final rinse of the "degreased" furnish with clean liquid solvent (usually the same as the solvent of which the hot vapor pool consists but optionally a different solvent). This step may often be omitted. PA1 (5) Removal of any residual solvent entrained in the stripped paperstock. PA1 (6) Recovery of any dissolved resins carried into the liquid solvent sump by the condensate, as, for example, by continually removing a bleed which is distilled (by which means any water present is also removed, usually as a lower boiling azeotrope). The resin is recovered as a bottoms product. Alternatively, dissolved resin may be separated by chilling and centrifuging or filtering or by other conventional separatory procedures. PA1 (7) Recycle of the recovered, water-free solvent (a convenient source of rinse-solvent for step (4)). PA1 1. The boiling point (121.degree./760 mm) of perchloroethylene is high enough to melt most hotmelt waxes. In contrast, methyl chloroform is not as effective in rapidly stripping hotmelt coatings because the coating must be dissolved in order to effect debonding thereof; that is, methyl chloroform has too low a boiling point to be very efficient; PA1 2. Perchloroethylene is a relatively poor solvent for polymeric hotmelts, thereby facilitating their recovery as a discrete second phase, such as a gel; PA1 3. The boiling point of perchloroethylene is low enough not to deleteriously affect the structure and physical properties of the cellulose fibers. Measurable properties of handsheets made from repulped fibers, such as freeness, burst factor and brightness, are unaffected by contact with this solvent; PA1 4. It has a very low heat of vaporization, the importance of which is explained above.
U.S. Pat. No. 2,413,144 discloses a method of removing waxes from shotgun shells (or other coated paper objects) wherein the shells (or objects) are individually suspended in hot vapors of a suitable solvent. No agitation is employed.